Match most appropriate base platform string based on insn_flags.
Logic is aligned with aligned with set_isa() from
arch/mips/kernel/cpu-probe.c in Linux kernel.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220803103009.95972-3-jiaxun.yang@flygoat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>